This invention relates generally to apparatus and methods for mixing and more particularly, but not by way of limitation, to apparatus and methods for mixing substances, such as a dry substance and a liquid substance like bulk cement and water, respectively, to form a cement slurry for use in the wellbore of an oil or gas well.
Both land based and offshore well drilling and completion operations often require on-site mixing of substances, such as cement slurries, acids and fracturing gels and weighting drilling fluids. In general, a mixing system includes a tub, pumps and various monitoring and control equipment. Also, an apparatus and method for introducing substances into the tub is part of the mixing system.
Prior art apparatus and methods of mixing include a jet mixer. A jet mixer typically sprays water under pressure into a venturi tube where bulk cement is added. The water and bulk cement combine to form a cement slurry, which is conveyed into a tub prior to pumping the slurry down a wellbore.
Another prior art mixer is an axial flow mixer such as that shown in U.S. Pat. No. 5,046,855, which is incorporated by reference herein. An axial flow mixer combines water, cement and recirculated slurry in an axial mixing tube to form a mixture. The prior art recognizes the benefit of adding recirculated slurry in order to increase the density and uniformity of the cement mixture. The mixture is then deposited in a tub prior to pumping the slurry down a wellbore.
While the prior art apparatus and methods for mixing provide satisfactory results, there are nonetheless shortcomings. For example, some prior art devices require numerous machined components which contribute to a complex device which is difficult and expensive to maintain. These parts must be frequently replaced due to the high wear associated with mixing a cement slurry. Furthermore, many prior art devices are large and bulky.
Moreover, the prior art devices inefficiently use the mixing energy which can result in an inconsistent mixture containing lumps of dry cement and pockets of air or air entrainment in the cement slurry. Additionally, incomplete mixing can cause cement dust to escape from the mixer.
Thus, there is a need for improved apparatus and methods for mixing which are cost effective, relatively inexpensive, low maintenance, simple and which effectively mix without dusting by efficiently using the available mixing energy.